Caption: English oak heartwood. Coloured scanning electron micrograph (SEM) of a horizontal section through the heartwood (secondary xylem) of the English oak (Quercus robur). A tracheary vessel (centre) has become blocked by papery bladder-like ingrowths known as tyloses. In young sapwood, the vessels carry water and nutrients from the roots to the leaves, but after a few years they become blocked and no longer carry sap. Large vessels like this occur in younger wood. The tissue surrounding the vessel is wood parenchyma, an unspecialised matrix tissue. Oak is classified as a hardwood. Magnification: x160 at 10 centimetres high.